Fabry-Perot filters are designed to pass narrow spectral ranges of light at a high efficiency with essentially no transmission of light of different wavelengths. Tunable Fabry-Perot filters are known wherein the spacing between the two plates is varied so as to pass radiation of different wavelengths as, for example, anywhere from 3 microns to 40 microns. U.S. Pat. No. 4,400,058 in the name of Ronald E. Peterson and William W. Durand and assigned to the assignee of the present invention describes and claims such a filter. A modification to this filter is found in U.S. Pat. No. 4,377,324 in the name of William W. Durand and Anil K. Jain and Ronald E. Peterson also assigned to the assignee of the present invention wherein the coatings on the plates have several layers with increasing indexes of refraction and the spacer thickness between the plates is decreased to far less than the design wavelength so as to produce a device which transmits a broader band in the vicinity of the design wavelength.
A tunable Fabry-Perot filter of an improved nature was also described and claimed in copending continuation-in-part application Ser. No. 467,619 filed Feb. 17, 1983 in the name of William W. Durand and Anil K. Jain and Ronald E. Peterson and assigned to the assignee of the present invention. In this application, three plates are used in the filter to provide, in effect, two Fabry-Perot filters, one of which passes a relatively broad band of frequencies such as was described in the above-referred to U.S. Pat. No. 4,377,324 and the other of which passes a rather narrow band of frequencies including their harmonics as was described in the above-referred to U.S. Pat. No. 4,400,058 so as to produce a filter which will pass only a rather narrow band of frequencies. As with the other applications, the spacing between the plates can be varied so as to select the desired wavelength.
All of the above-referred to prior art concerned itself with radiation which was passing with a plane wave front and the plates of the filter were accordingly planar. This was necessary so that the spacing between plates and the angle of incidence of the radiation to the plates would be maintained substantially perpendicular to the oncoming radiation.
It has been found desirable to employ a Fabry-Perot filter for use in an optical system that focusses energy, such as in an imaging system, in which event the oncoming radiation may have a sperical wave front and be focussed at some point along the optical axis. The use of a Fabry-Perot filter with planar plates would provide for inferior operation since the angle of incidence upon the plates would vary anywhere from perpendicular at the optical axis to some angle at a position off the optical axis. Accordingly, in such a system it has been considered desirable to curve the plates of the filter in such a way as they would be concentric about the point of focus of the energy. By doing this, the rays upon the surface of the plates remains perpendicular and the distance between the plates is constant. In a U.S. Pat. No. 3,775,699, issued Nov. 27, 1973, in FIG. 4, a laser mode selector is shown in which the wave front is spherical and accordingly a Fabry-Perot etalon with concentric spherical surfaces is used. A concentric spherically shaped Fabry-Perot filter is satisfactory for one specific spacing between the plates but when, as with a tunable filter, the plates are moved with respect to one another, the spacing between the plates changes in such a manner as to produce inferior operation of the filter at other than the one specific wavelength for which it is set.